1. Field of the Invention
The present invention relates to a feeder apparatus and method for dispensing particulates. More particularly, the present invention relates to apparatus and methods for controllably drizzling both free-flowing dry particles and particles which may not be entirely dry nor truly free-flowing (i.e., particles which may be somewhat moist, sticky, tacky, or cohesive so that they may tend to aggregate or clump) from a bulk sample of the particles into an air stream conveying the particles through an analysis cell of a particle analyzer. In the analysis cell, optical methods including light back scatter, forward scatter or diffraction, and obscuration effects are employed to interrogate the particles for certain physical characteristics, such as particle size distribution of the bulk sample.
2. Related Technology
Present dry particle analyzers generally use two methods to feed particulate materials from a bulk sample of the material into a conveying air stream. For materials which are not too free flowing (i.e., are moist, tacky, or tend to aggregate), the feeder includes an auger metering particles from a container for the bulk sample into a vibratory trough. The vibratory trough employs a downward incline in addition to a selectively variable level of imposed vibration to deliver the particles to an end of the trough which is suspended over the mouth of an air intake duct. When the particles drop from the trough end, a vacuum system draws the particles and a conveying ambient air stream into the air intake. Apparently, the thinking behind this type of feeder is that the vibration of the duct will break up any agglomerations of the particles, and that a technician can control the turning rate of the auger and vibration rate of the trough to produce a desired rate of particle feed from the bulk sample.
Experience with the auger and trough feeder has shown, however, that if the particles tend to form agglomerations, they arrive at the end of the trough pretty much like they entered it. That is, with some materials the agglomerations are not in fact broken up by the vibrations of the trough, and the material enters the analyzer in clumps and bunches rather than as a steady drizzle. Of course, this inconsistent delivery of the particles to the analysis cell detrimentally affects the accuracy of testing.
An important aspect of this type of particle testing is control of the obscuration effect of the particles in the analysis cell. That is, control of the extent to which the flow of particles in the air stream blocks light from passing through the cell is fundamental to this type of particle testing. An inconsistent flow of particulates to the cell makes it virtually impossible to accurately control the light obscuration and adversely affects the test accuracy. Additionally, to the extent that the skill of the technician influences the sample feeding rate, and the resulting accuracy of the particle test, test accuracy and repeatability of particle analysis tests conducted with this type of sample feeding might be in question.
Another type of particle feeder primarily for particles which are truly dry and free-flowing includes a sample cup having an upper chamber floored with a sieve screen. Below the sieve screen, the sample cup includes a funnel-like portion with a spout leading to the mouth of the air intake duct. When the sample cup is vibrated or oscillated, the particles sift through the sieve screen and drizzle out of the spout of the funnel-like portion to be conveyed by the air stream to the analysis cell of the particle analyzer.
Unfortunately, the conventional sieve cup tends to classify some particle samples even as the sample is being fed into the particle analyzer. That is, the sieve screen needs to be fine enough to hold back the bulk sample while no vibration or oscillation is being applied to the sample cup. In other words, the sieve screen can not be so coarse that the bulk particulate sample simply flows through like a liquid. In fact, the sieve screen needs to be additionally fine enough to also hold the fines of the sample as part of the bulk while not agitated. The lower portion of the sample immediately above the sieve screen would otherwise be partially depleted of these fines before the test could begin. On the other hand, the sieve screen can not be so fine a mesh that the coarse particles of the sample will not sift through even under agitation.
Regardless of the mesh size of sieve screen selected, while a bulk sample is being drizzled with controlled vibration to a particle analyzer, the fines of the sample may tend to migrate downwardly through the coarse particulates, and to pass freely through the sieve screen. At the same time, the coarser particulates may tend to be held back at the sieve screen and to sift through less readily. As a result, when vibration is initially applied to the sample cup to begin drizzling the sample to the analyzer, the initial portion of the bulk sample drizzled to the analyzer may be representative of the bulk. However, an interval may then follow during which a second part of the sample which is rich in fines is drizzled to the analyzer. As these fines migrate to the sieve screen through the bulk of the sample they pass immediately through the sieve screen ahead of coarser particulates and leave a remainder of the bulk sample which is depleted of fines. Finally, the remainder of the sample, rich in coarse particulates and depleted of fines is drizzled to the analyzer. The result is a time-variant classification of the bulk sample which may result in a decreased accuracy of measurement of the particle size distribution of the sample. Thus, a design paradox is presented to those who would use a simple sieve screen to feed a drizzle of particles to a particle analysis cell. No matter the sieve mesh chosen, a detrimental effect may be encountered with some particulate materials.
Additionally to the above, regardless of whether a conventional sieve screen sample cup or the Applicants' new type of sample cup as described herein is used to drizzle particulates, the Applicants have discovered that the nature of the applied vibrations has a considerable effect on the feeding of the particles. That vibration in a horizontal plane, whether purely linear reciprocation, tapping, Jiggling, orbital, or other motion, simply rolls the particles around on the upper side of the sieve screen, and provides no driving force to assist the particles through the sieve, beyond that already present from gravity. On the other hand, if a vertical shaking or reciprocation is applied to the particles, this motion simply tends to tamp or pack down some materials into a plug which will not thereafter drizzle as desired. This tamping or packing effect of purely vertical dispensing motion seems to affect all sample cup designs. Such is especially the case with materials like corn starch which tend to clump. Also, the rate of feeding of particles out of the bulk sample and into the particle analyzer may not be proportionate with the vibration rate. In some cases with some materials, the rate of particle feed may actually decrease with an increased vibration rate. Such an inverse feed rate relationship makes control of test conditions very difficult.